1. Field of the Invention
The present invention relates to an X-ray Computed Tomography (CT) apparatus and a tomography method for reconstructing an image by irradiating a subject, switching scanning conditions, with X-rays continuously or intermittently based on a plurality of sequential scanning plans and detecting X-rays that has passed through the subject. Particularly, the present invention relates to an X-ray CT apparatus and a tomography method by which a scan can be performed with an X-ray dose optimal for each subject.
2. Description of the Related Art
Recently, along with prevalence of an X-ray detector of multiple arrays, an X-ray CT apparatus that can take a four-dimensional image that one temporal dimension is added to three spatial dimensions has been developed. With such X-ray CT apparatus, motions of an organ or blood flow dynamics can be observed temporally, therefore, the X-ray CT apparatus can be used when performing a perfusion examination of an orthopedic region or various organs for a functional diagnosis, as well as a conventional examination for shape diagnosis.
Usually, an ordinary X-ray CT apparatus performs a scan after an operator preliminarily sets a plurality of scanning plans including various scanning conditions (for example, X-ray irradiation intervals, a scan time, and a tube current to be supplied to an X-ray tube). When taking an image for the perfusion analysis, to reduce an exposure dose to a subject, it is desirable that scanning plans are set in varying X-ray irradiation intervals in accordance with the density of a contrast medium in blood flow rather than performing a scan with regular intervals through to the end, so that scanning plans with different irradiation intervals are used in combination.
For example, when taking an image for a perfusion analysis for 60 seconds, a method of performing a continuous scan for 60 seconds may be available; however, from an exposure-dose point of view, a scan is performed by combining two or three scanning plans. Specifically, scanning plans are set as follows: because detailed data is required from the start of a scan to a peak of the density of a contrast medium, a continuous scan or an intermittent scan with short intervals is performed; a moderately intermittent scan with slightly longer intervals (for example, two-second intervals) is then performed in the middle stage during which the density of the contrast medium gradually decreases after the peak; and then an intermittent scan with much longer intervals (for example, three-second intervals) is performed in the final stage during which almost no change is observed in the contrast medium.
In this way, the conventional X-ray CT apparatus performs a scan by setting scanning plans based on a prediction of temporal variations in the density of a contrast medium. However, speeds of the circulation of a contrast medium through a body vary from subject to subject, therefore, a scan time of each scanning plan has no other choice but to be set rather long in order to perform a scan without missing a period during which the density of the contrast medium changes markedly. As a result, a subject may be sometimes irradiated with unnecessary X-rays for the perfusion analysis in some cases, for example, due to a continuous scan or an intermittent scan with short intervals that is continued even after the density of the contrast medium has peaked out in practice, or due to an intermittent scan with medium intervals that is continued despite that change in the contrast medium is hardly observed.
For this reason, it is required to perform a scan with a dose optimal for each subject by controlling the start/end of scanning plans automatically in accordance with the density of the contrast medium during the scan. As a technology of determining start timing of a scan in accordance with the density of a contrast medium, a technology is invented such that before a scan for taking an image for diagnosis (main scan), a preparatory scan (prep scan, also called “Real Prep” or “Sure Start”) is performed, and then start timing of a main scan is controlled based on a temporal variation curve of CT values measured through the prep scan (for example, see JP-A 2007-275360 (KOKAI)).
However, according to the conventional technology represented by JP-A 2007-275360 (KOKAI), even though start timing of a main scan can be controlled; after the main scan is started, i.e., during the scan, the start/end of a scanning plan cannot be automatically controlled in accordance with the density of the contrast medium.